The Average Response Time (per day for example) is an average of the individual response times measured during that day. So each time we check your web site (or URL) we record the time it takes for the server to respond to our request. This is the first byte response time. Then we average that out for the day.
For instance, .605 equals .605 seconds (or 6/10ths of 1 second),and is the average response time from your server for our requests. We are not downloading all the content of the web page (like images, videos, etc...) so it is going to be a quicker response time then maybe a browser would get. But it would still be proportional; if we get a slow response time then users browsing to the site probably are finding it slow to load as well.